The present invention relates to power systems and more particularly to power systems that generate regulated power for use in electrical load switches including, for example, lighting system switches.
Commercially available units have been designed to replace existing wall switches in commercial and private applications. These units typically include load switching devices that replace the existing mechanical switch contacts that are used to switch the electrical load. These load switching devices may include relays, SCRs Triacs, transistors, or other electrical load switching devices that may be controlled by power control circuitry including, for example, a programmable controller, or the like. Many of these replacement units require a power supply for the power control circuitry that must supply power to the control circuitry whether or not the load switching device is in the on-state or the off-state. The wiring that exists in the existing switch enclosures, the mechanical constraints imposed by the existing switch enclosures, and the constraints presented by the existing loads cannot be easily altered and must be tolerated by the unit that is replacing the existing switch.
Units that have been designed as replacement devices for existing switches range from simple dimmer switches to intelligent lighting systems with microprocessor control. Viable commercial products that may be used around the world in business, as well as in private locations, require replacement units that are low cost, robust, meet stringent safety considerations, are small in size, have low loss and have attractive physical features.
Existing load switches are either three-wire, or two-wire systems. Three-wire systems require hot, load and safety ground wires, whereas two-wire systems require only hot and load wires. In three-wire systems, although not required for proper operation, a line voltage potential is present within the switch enclosure whether the contacts of the switch are open or closed, while in a two-wire system, voltage is present within the switch enclosure when the contacts of the switch are open, but no voltage or a very reduced voltage is present when the contacts of the switch are closed. In addition, three-wire systems usually place constraints on the amount of current that may flow in the safety ground. For two-wire systems, the safety ground is not required for operation but there are constraints on the amount of current carried by the load wire when the contacts of the switch are open. It is desirable to provide a power system useable in either a two-wire or three-wire system, with little if any circuit changes.
With two and three-wire systems, supplying more than 15 mA to the switch control electronics when the switch contacts are open while at the same time limiting current to the load to less than 3mA has been difficult to achieve in a very small package size.
FIG. 1 is a simple illustration of prior art two-wire and three-wire systems and a wall switch enclosure through which a source AC voltage 1 and a load 2 have been wired.
In a two-wire system, the wall switch enclosure 37 has only the two wires hot conductor 3 and load conductor 4, while in a three-wire system, safety ground 38 also appears within enclosure 37.
There presently exist many two-wire load switch replacement units. Most of these units add a transformer primary winding in series with the relay contacts so a portion of the input power may be extracted by magnetic coupling to a secondary winding. The inserted winding adds a voltage insertion drop in series with the load whenever it is conducting load current. Since it is undesirable to reduce the voltage available to a load in most instances, designs in this class try to keep the insertion voltage drop to a minimum. If the voltage drop is small with respect to the line voltage, and thus the current that flows in the primary winding of the inserted transformer is not dependent to a large extent on the inductance of the primary nor the reflected impedance from the secondary when loaded, the primary may be said to be driven by the load current.
The transformers used to derive power while the load is energized have a low insertion voltage requirement for the primary dictated by Ldi/dt; where L is the inductance of the primary and di/dt is the time rate of change of the input current. Ldi/dt is insertion voltage and as such should be low enough when di/dt is large to not appreciably affect the operation of the load and large enough when di/dt is small to provide adequate power transfer to the secondary. Therefore, limiting the inductance L of the primary and providing adequate turns ratio so the secondary voltage is compatible with the selected regulator type is what dictates the transformer constraints. The di/dt term is determined by the frequency of the source line voltage and the load current. Prior art has been directed to a transformer that does not appreciably affect the current to the load. Among other disadvantages of these prior approaches, these devices have typically been too large to fit the strict size requirements for replacement switches.
It would be preferable for the replacement device's contacts to emulate the simple mechanical air gap switch it replaced. The replacement device should also have a similar voltage drop when the contacts are closed, essentially zero leakage current when the contacts are open, and a guarantee of safety from hazardous voltages when the contacts are open. Prior devices do not provide all of these desirable advantages.